(1) Field of the Invention
This invention relates to a waveguide structure for a semiconductor, and more particularly, to a waveguide structure made of the II-VI group compound comprising a II group element (e.g., Zn, Cd, Mg, Hg, etc.) and a VI group element (e.g., O, S, Se, Te, etc.).
(2) Description of the Prior Art
In the prior art, a wide variety of structures has been suggested and produced for waveguides of the III-V group compound semiconductor.
However, in such III-V group compound semiconductor, heretofore it has been impossible to guide a blue/green beam to violet beam of wave lengths shorter than approximately 0.54 micrometers.
Also in the prior art, a waveguide structure as shown in FIG. 1 has been suggested for the II-VI group compound semiconductors. In FIG. 1, reference numerals 101 and 105 designate clad layers having a composition of ZnS.sub.0.07 Se.sub.0.93, 102 and 104 waveguide layers of ZnSe, and 103 a waveguide layer of Zn.sub.0.8 Cd.sub.0.2 Se. Each of the clad layers 101 and 105 has an approximately 2 .mu.m in thickness and the thickness of the waveguide layer 103 is 0.1 .mu.m. Note that, both GaAs buffer layer 106 and ZnSe buffer layer 107 are formed between a GaAs substrate 201 and the clad layer 101 (M. A. Haase et al, Appl. Phys. Lett., 59 (1991) 1272).
Besides the above construction of the II-VI group compound semiconductors, there is known a waveguide which employs a multi-quantum well of Zn.sub.0.08 Cd.sub.0.12 Se-ZnS.sub.0.07 Se.sub.0.93 as the waveguide layer while using the clad layer of ZnS.sub.0.07 Se.sub.0.93 (H. Jeon et al, Appl. Phys. Lett., 59 (1991) 3619) and another waveguide which employs a multi-quantum well of Zn.sub.0.91 Cd.sub.0.09 Se-Zn.sub.0.95 Cd.sub.0.05 S.sub.0.07 Se.sub.0.93 (K. Ichino et al, Oyo Buturi (Japanese Journal of Applied Physics), 61 (February 1992) 117).
In the II-VI group compound semiconductor containing the elements Zn, Cd, S and Se, it is impossible to form a waveguide having a large difference in refractive index between the clad layer and the waveguide layer any yet have it satisfy a lattice alignment condition. Therefore, there is known a conventional waveguide structure which contains a lattice inalignment structure (a difference of lattice constant) between the clad layer and the waveguide layer. For example, in the above-mentioned conventional structure of FIG. 1, there is contained one lattice inalignment of 0.24% between the layers of ZnS.sub.0.07 Se.sub.0.93 and the layers of ZnSe and the other lattice inalignment of about 1.5% between the ZnSe layers and the layer of Zn.sub.0.8 Cd.sub.0.2 Se. The refractive indexes of the layers of ZnS.sub.0.07 Se.sub.0.93, ZnSe and Zn.sub.0.8 Cd.sub.0.2 Se exhibit 2.67, 2.69 and 2.83, respectively, to the beam having a wavelength of 530 nm. Therefore, the difference in refractive index between the clad layer and the waveguide layer is small, and particularly small between the layers of ZnS.sub.0.07 Se.sub.0.93 and the layers of ZnSe. Consequently, in the II-VI group compound semiconductor laser, the function to confine the beam therein is remarkably small.
As mentioned above, in the conventional waveguide structure of the II-VI group compound semiconductor, if the difference in refractive index is increased between the clad layer and the waveguide layer to effect the beam confining function, there would be produced a large lattice inalignment therebetween, whereby it would be difficult to form a crystal thin film of fine quality.